Views: 66 Author: Site Editor Publish Time: 2021-09-06 Origin: Site
Driven by foundries and brand owners, it is estimated that by 2024, the global production capacity of bioplastics will increase to 4.8 billion pounds (approximately 2.18 billion kilograms), so that more and more applications will adopt such green materials. For compounders, how to compound such materials with high quality on a twin-screw extruder?
Biopolymers such as PHA will drive the growth of demand for green materials. Some sources predict that the production capacity of such materials will increase by more than three times in the next five years. PHA is a biodegradable bio-based polyester with good physical and mechanical properties.
In the field of biopolymers, many new participants have also emerged, and they have brought new materials and many new formulations.
Including biopolymers, all polymers can be produced using co-rotating twin-screw extruders, which have excellent material mixing capabilities. Regardless of the formula, if you want to obtain excellent melting and mixing quality, increase yield, and achieve other parameter goals, you should consider the machinery supplier as a development partner, and ensure that the selected machinery supplier has the equipment to help you test The facilities and expertise for material formulation and process improvement are because, as with any compounding project, a certain amount of trial and error will inevitably be carried out in the process of developing a process for a new biopolymer formulation.
Biopolymer compounding materials can be roughly divided into three categories: starch-based polymer compounding materials; bio-resin compounding materials containing bio-resin, such as PHA, PLA or PHB; biopolymer compound blends, which contain two types Or above bio-resin, starch-based polymer and/or petroleum-based polymer compounding material.
When producing bio-resin compounds and bio-polymer compound blends on a co-rotating twin-screw extruder, the following key items need to be paid attention to:
1. Buy a flexible extruder. This is a very fast innovation industry, with new ideas, new or improved raw materials, and new or improved additives. Therefore, the design of the extruder should ensure maximum flexibility. It is best to purchase a long extruder with a length-to-diameter ratio of 48:1 or 52:1, and multiple vents and feed location options should be provided. Opening or closing the available vent locations, or adding or removing a side feeder from an available location, is much easier than purchasing a new barrel or expanding the extruder after the initial purchase.
2. Remember that biopolymers are usually hygroscopic. Therefore, at least one exhaust port is required, and sometimes two are required. At the same time, at least one vacuum exhaust port is required, and sometimes two are required. In short, because biopolymers are generally hygroscopic, they need to be handled carefully before processing. Setting a vent in the early stage of extrusion process can minimize the hydrolysis of the biopolymer and avoid pre-drying of the material. It is also necessary to set up a vacuum exhaust port, do not regard the vacuum system as an unnecessary accessory, it is an indispensable part of the extrusion process. A well-designed vacuum system can also solve the corrosive problem caused by the volatilization of biopolymers, so that the cost can be recovered as soon as possible by reducing maintenance and downtime and increasing production.
3. Please consider the sensitivity of biopolymers to shear and pressure. When exposed to excessive heat and/or shear, biopolymers will rapidly degrade. The shear force experienced by the material is directly proportional to the screw speed of the extruder and inversely proportional to the gap between the screw and barrel of the extruder. Therefore, it is necessary to run the extruder at the lowest possible screw speed under a given extrusion volume, until the extruder torque is too high (>90%) or the volume limit is reached, which can block the feed port through mixing Or exhaust port to determine.
In order to avoid volume limitation, the exhaust port can be strategically set to exhaust air and moisture, and the restricted kneading section or the reverse screw element can be placed as far downstream as possible from the feed port or the exhaust port. By designing the extruder screw to have a longer "soft" mixing zone instead of a shorter vigorous mixing zone, high torque can be avoided. Stabilizing the feeder to minimize torque fluctuations allows the operator to run the extruder with a higher average torque.
Although the co-rotating twin-screw extruder is a high-efficiency compounding equipment, its pumping efficiency is only 8% to 15%. The conversion effect brought by this is that the melt temperature is caused by the high pressure at the outlet of the extruder Significantly higher. The higher outlet pressure also results in an exhaust flow at the exhaust port near the exit of the extruder. Increasing the die hole diameter, increasing the mesh size of the filter, and in the case of underwater pelletizing, increasing the temperature of the water and the die, all of these will minimize the outlet pressure of the extruder. A conveying screw element with a pitch equal to one diameter is the most effective pumping element.
A melt pump can be used between the extruder outlet and the die. The pumping efficiency of the melt pump is 25% to 35%. The use of a melt pump can minimize the excessive increase in melt temperature, save the power of the extruder and form a more stable extrusion flow. Sometimes, the rationality of the melt pump cost can be verified by increasing the output of the extruder.
4. Biopolymers degrade rapidly, therefore, cleaning the machine is very important. Biopolymers degrade faster than most petroleum-based polymers. If the extruder is to be shut down for more than 1 to 2 hours, it is strongly recommended to reduce the heat of the barrel or turn off the heating. If the extruder is kept heated for a long time when it is not running, please use low-cost heat-stable polymers to clean the extruder. The melt viscosity of this type of polymer is slightly higher than that of bio-resin. If the viscosity of the cleaning material is too high, it will show up as a contaminant within many hours after restarting production.
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